Size reduction of metal-oxide-semiconductor field-effect transistors (MOSFET) has enabled the continued improvement in speed, performance, density, and cost per unit function of integrated circuits. One way to improve transistor performance is through selective application of stress to the transistor channel region. Stress distorts (i.e., strains) the semiconductor crystal lattice, and the distortion, in turn, affects the band alignment and charge transport properties of the semiconductor. By controlling the magnitude and distribution of stress in a finished device, manufacturers can increase carrier mobility and improve device performance.
One approach of introducing stress in the transistor channel region includes growing an epitaxial layer of SiGe within recesses in the source/drain regions. In this case, lattice mismatch creates a uni-axial compressive stress within the channel region. One problem facing complementary metal-oxide-semiconductor (CMOS) manufacturing is that N-channel metal-oxide-semiconductor (NMOS) and P-channel metal-oxide-semiconductor (PMOS) devices require different types of stress in order to achieve increased carrier mobility. PMOS fabrication methods may include using substrate structures that apply a compression stress to the channel. Therefore, CMOS manufacturing techniques may address PMOS and NMOS devices separately.
High germanium concentration in epitaxial silicon germanium (e-SiGe) may be needed to boost channel compressive strain in PMOS devices. Boron doping may be incorporated into the e-SiGe for lower sheet resistance and contact resistance in the source drain regions. However, because of the high concentration of boron in the e-SiGe, the boron may tend to out-diffuse into the channel region. Boron out-diffusion in a PMOS may result in a voltage threshold (vth) reduction in short channel transistors. This roll-off in vth is termed the short channel effect (SCE).
To counteract the effects of the boron out-diffusion, one known method includes a series of implants (termed pocket or halo implants) following the etching of source/drain recess regions and dummy sidewall regions of the gate electrode. The pocket implants may have a phosphorous dose of about 4.0 E13 cm−2 or greater and an arsenic dose of 3.0 E13 cm−2 or greater. One disadvantage to the known method is that the pocket implant dose may be high enough to cause damage to the sidewalls and bottom of the recessed source/drain regions, in which the stressor material layer or layers are deposited. A further disadvantage to the known methods and structures is that boron out-diffusion is not adequately controlled, resulting in short channel effects.